Various types of semiconductor devices are manufactured in much the same way. A starting substrate, usually a thin wafer of silicon or gallium arsenide, is masked, etched, and doped through several process steps, the steps depending on the type of devices being manufactured. This process yields a number of die on each wafer produced. The die are separated with a die saw, and then packaged into individual components.
During the plastic packaging process, several semiconductor die are attached to a lead frame, often with a material such as metal, or an epoxy or other viscous adhesives. Bond wires couple each of several terminals (bond pads) on each die to conductive leads on the lead frame. The die, the wires, and a portion of the leads are encapsulated in plastic. These leads couple the die with the device into which the component is installed, thereby forming a means of input/output (I/O) between the die and the device. A die is encased in ceramic by attaching the die to a shelf in the preformed package, then bond wires couple bond pads on the die with attachment points on a second shelf of the ceramic package. Traces couple these points on the second shelf to the exterior of the package. Leads can be coupled thereto to allow for I/O between the die and the device into which the package is installed.
One step of semiconductor manufacture that is not without problems is the die-lead frame attachment. During the manufacturing process, several die are attached to a length of lead frame at separate locations along the lead frame. Bond wires are then connected from the bond pads on each of the die to the "fingers" on the lead frame, after which each die is encapsulated in a protective plastic casing. The plastic packages are separated, and the leads are formed into a desired shape.
The lead frame, part of which will eventually form the conductive leads of the components, contains a major surface to which the die is attached, called the "paddle." The die is normally bonded to the paddle with epoxy or metal, although thermoplastic, tape, or another materials are also used. The paddle, which covers the entire bottom surface of the die, is connected to the lead frame with a tie bar. The tie bar, which is connected at either end to one rail of the lead frame, is typically severed after encapsulation of the die to separate the paddle from the lead frame, the tie bar serving to support the paddle and the die attached to the paddle before encapsulation.
FIG. 1A is a top view, and FIG. 1B is a cross section, showing a conventional lead frame 10 having die paddles 12 with die 14 attached. The frame 10 comprises dam bars 16 which restrict the flow of encapsulating material during encapsulation, exterior leads 18 which are unencapsulated, and lead fingers 20 which will be encapsulated. Bond wires 22 electrically couple bond pads 24 on the die 14 with the lead fingers 20. The adhesive 26 used to attach the die 14 to the lead frame 10 is dispensed on the die paddle area 12 of the lead frame 10. The die 14 is placed on the uncured epoxy (for example) and held at a specific pressure by die attachment equipment having a surface contact tool or an edge contact only tool (collet). The die is pressed down into the adhesive at a specific pressure, and often at a controlled temperature, by the tool and held in place long enough to ensure adhesion. X-Y movement (scrub) is sometimes used to increase adhesion and to speed the process. The attach process often requires a follow-on cure in a separate cure oven. After the attach process, the assembly within the dam bars 16 is encapsulated. The paddle 12 of the lead frame 10 is usually at a lower plane 28, which allows better control of the plastic encapsulation material as it is being injected into the mold. This lessens the chance, for example, of the bond wires 22 detaching from the lead fingers or bond pads. The paddle 12 is connected at either end to the rails 30 of the lead frame 10 by a pair of tie bars 32.
Various problems are associated with the connection of the die to the die paddle. Occasionally a corner of the die will crack, thereby making the semiconductor inoperable. This can result from stress placed upon the die by the adhesive due to an uneven thermal coefficient of expansion between the die and the adhesive. After the die is attached to the lead frame and oven cured, the assembly is heated at the wire bond step to attach the wire to the die pad. If the die and the adhesive expand at different rates, undue stresses can be inflicted on the die. Cracks can also occur from stress on the die due to shrinkage of the adhesive as it cures, although in recent years chemical improvements in adhesive has reduced this cause of cracking.
The paddle itself also creates extra expense. The package leads are typically gold or silver plated, and the die paddle is also plated along with the conductive leads, in part because the paddle cannot economically be masked during plating of the conductive leads. The leads are plated to provide the proper metallic surface to which to wire bond since the bond wire will not stick directly to the material usually used for the lead frame, such as copper or alloys. The plating of the paddle, however, serves no functional purpose. This unnecessary gold or silver plating of the paddle, which is a relatively large surface, adds unnecessary cost to the product.
Occasionally the die and epoxy may come loose from the lead frame, a problem referred to as "popping die." Popping die can result from too little adhesive under the die, a poor bond between the adhesive and the paddle, or from bowing of the die paddle from heat or pressure. This can be a serious problem, not only because it results in scrapping the die but also because the loose die can damage the molds which are used to encapsulate the package.
One way to solve the problems associated with the lead frame paddle is described in U.S. Pat. No. 4,862,245 by Pashby, et al. which is incorporated herein by reference. Pashby, et al. describe a die which allows for a "leads over die" configuration, in which the leads are attached to the top of the die, then the bond pads are coupled with the lead fingers by bond wires. A leads over die configuration has the advantage of not requiring a die paddle. It also allows for a smaller package, as well as other advantages described therein.
One disadvantage of the die described in Pashby, et al. is that it is difficult to incorporate into a conventional ceramic package. FIG. 2 shows a conventional die 14 in a ceramic package 40 having a desirable length of bond wires 22, and FIG. 3 shows a die 50 of Pashby, et al. in a similar package 40, which requires excessively long bond wires 52 to couple bond pads 24 in the center of the die 50 to the attachment points 54 on the second shelf. Long bond wires are known to have reliability problems, for instance under high gravitational forces. A manufacturer would need a pair of die designs, one for a plastic "leads over die" configuration, and one for a ceramic package.
Additional die designs are typically avoided by a semiconductor manufacturer wherever possible. A lead frame design which does not require a die paddle and which allows the advantages of a leads over die configuration using a conventional die design would be desirable.